This invention relates generally to cages for the accurate measurement of dimensions or distances on the basis of capacitance or its variations. More specifically the invention deals with a capacitive probe assembly for such precision dimensional or distance gages.
The probe of a high precision capacitive gage usually takes the form of a concentric arrangement of an electrically conductive housing, a measuring electrode, and a guard electrode. The conductive housing is held at ground potential for shielding the measuring electrode from the influences of external stray capacitances. The guard electrode functions to correct distortions in electric lines of force due to the presence of the grounded housing. The electrodes are exposed through one end of the housing, and this end of the housing, together with the exposed ends of the electrodes, is machined to an exactly planar, mirrorlike finish for the minimization of measuring errors.
As the planar sensing end of the probe is held opposite to a reference electrode, the measuring and reference electrodes combine to make up a capacitor. Thus the variations in capacitance between the electrodes can provide indications of the distance therebetween or of the thickness of an object intervening therebetween.
Stringent requirements are imposed today on capacitive probes. They are required to detect extremely small dimensions or distances or changes in them. Their capacitance-sensing abilities must be several ten times as high as those of the conventional ones. Such high sensitivities are attainable only by making the probes immune to stray capacitances and other environmental influences. Another necessity is to prevent the occurrence of irregularities in the planar sensing surface of the probe, which is made up of materials having different coefficients of thermal expansion, despite changes in temperature and other conditions of use.
These requirements are met to some extent by the probe described and claimed in U.S. Pat. No. 3,805,150 issued to Abbe on Apr. 16, 1974. It has one or more measuring electrodes and one or more balancing electrodes enclosed in an electrically conductive housing which is held at ground potential. Electric circuit means associated with the measuring and balancing electrodes are also contained in the housing. The grounded housing can shield the electrodes and the circuit means from external stray capacitances. A problem common to this and other known types of capacitive probes, however, is the difficulty of assemblage. Moreover, as far as the applicants are aware, there have been suggested no probe constructions providing for the interchange of electrode sections and circuit sections.